In order to provide relatively low cost, inertial grade sensors for measuring angular rate of rotation as well as linear acceleration, sensors have been developed that include a pair of accelerometers and their mounting structure fabricated out of a monolithic planar substrate. The accelerometers can then be dithered along the plane of the substrate so that the output signals of the accelerometers have a Coriolis component which can be used to measure angular rate of rotation. The accelerometers can also provide signals representing linear acceleration of the sensor in an axis normal to the plane of the substrate. Typically, the accelerometers and mounting structure are etched or micromachined out of a monolithic wafer of silicon. Examples of this type of sensor are described in U.S. Pat. Nos. 5,241,861, 5,319,976, 5,331,853, and 5,341,682 all of which are assigned to the assignee of this invention.
It has been discovered that small angular alignment errors, on the order of 100.mu. radians, can occur due to accelerometer misalignment resulting from minute manufacturing tolerances and inherent deviations in wafer flatness. Since rate accuracy in this type of sensor is a function of full scale g-range, dither frequency and phase resolution, a 100.mu. radian error in a sensor being dithered at 600 Hz can result in a bias uncertainty of 3 degrees per hour. One approach to solving this problem is described in U.S. Pat. Nos. 4,665,748 and 4,799,385, both of which are assigned to the assignee of this invention, where accelerometer misalignment is reduced by measuring the in-phase component of the dither signal and serving a parallelogram mechanism that supports the accelerometers. However, such a parallelogram accelerometer support mechanism is not practical in the type of sensor where the accelerometers are fabricated out of silicon wafer.
Also, the sensors described in U.S. Pat. Nos. 5,241,861, 5,319,976, 5,331,853, and 5,341,682 can be subject to cross-axis linear vibration sensitivity because the hinges supporting the proof masses of the accelerometers are located on opposite sides of the substrate.